Since the advent of Cable Television a variety of methods have been proposed and implemented to secure Pay Television against unauthorized access.
The present practical security systems offer both active and passive systmes which are designed to prevent non-subscribers from watching a Pay T.V. Premium Channel. In a passive system the signal is filtered out by a trap at each non-subscriber's location; in an active system the signal is mutilated or coded such that only a subscriber with a decoder supplied by the Pay T.V. company can restore the signal such that it can be viewed.
The traps in a passive system are typically mounted at the take-off point of the cable to the subscriber. Those traps are often illegally removed or tampered with by other means. Each non-subscriber needs a trap, resulting in a high capital investment for those Premium programs which have a relatively low saturation. It is further physically impossible in practice to remove or add traps just for a Pay-Per-View programs, for which the subscriber has to pay a special charge. In an active system the signal is scrambled before it is distributed. Two types of active systems are in use. In one type the subscriber receives from the Pay T.V. company a decoder which decodes the signal all the time. In the second type, either by addressable decoders or by other means the subscriber can only watch those specific programs for which he pays.
It has been proven very difficult to design a secure TV coding system for a reasonable cost. As a result, in many systems simple decoders can be bought, made by entrepeneurs not related to the Pay T.V. company, which decode the signal such that the viewer does not have to pay the Pay TV Company. In other systems which are more secure, the decoders are extremely complex and therefore very expensive.
Another complication arises in the case of Pay-per-View programs. In that case the Pay TV Company has to install an expensive decoder in the home of each potential customer, without any assurance that the customer will ever buy a program. This requires a very large capital investment by the Pay-T.V. operator.
This invention relates specifically to active security systems and the invention offers a solution to provide secure decoders at a low cost with the added advantage that Pay-per-View programs can be easily accommodated with a minimum investment by the Pay TV company.
In one particular active system the synchronization signals are modified such that a regular television receiver cannot synchronize to the signal. The disruption of synchronizing signals is inadequate because it may be easily circumvented. A technician of moderate ability can construct a decoder to stabilize the synchronization of the receiving television set.
Systems which inject an interfering or scrambling signal between the video and audio carrier are also known. Such systems are disclosed and described in U.S. Pat. Nos. 3,896,262 and 4,074,311 incorporated herein by reference. The advantage of the system of the latter patent is that it is very effective in interfering both with the video and audio carrier to prevent reception of a usable television signal at the user or subscriber end. An additional advantage of this system is the low cost of the decoder needed for removing the interfering or scrambling signal. However his system suffers from the disadvantage that the decoders, though inexpensively produced, can be also produced by pirates with only moderate technical knowledge and ability. The decoders are easily reproduced since they use circuitry made up of standard components, which can be easily obtained, to remove the interfering signal. This restriction is the result of the necessity of placing the scrambling signal a sufficient distance from the video and audio carriers to allow removal without removing a substantial amount of, and degrading, the video signal. The reason for this is that circuitry made up of standard discrete components can only produce an inefficient notch filter which will remove too much of the video information if the interfering carrier is too close to the video signal. The system of the latter patent is thus limited to using an interfering carrier near the center of the band between the video and audio carriers.
Pirating of scrambled T.V. signals is a serious problem in the Pay T.V. industry. There is at present no economical fully secure system and the need remains for a secure, economical Pay T.V. Transmission system to make Pay T.V. and particularly "Pay-per-View" a desirable business proposition.
Noting that an economical, near fully secure Pay T.V. system would be an outstanding advance in the industry it is therefore one object of the present invention to provide a low cost television signal transmission security system which may not be circumvented using standard, easily available components.
Another object of the present invention is to provide a television signal transmission system secure against pirating by even skilled artisans.
Still another object of the present invention is to provide a restricted access television signal transmission system in which the technology for the system decoders is valid even beyond VHF frequencies.
Yet another object of the present invention is to provide a secure television transmission system utilizing decoder technology which permits sophisticated coding techniques to be used. The coding techniques used involve multiple different interfering frequencies at different times or from program to program. Decoding efficiency allows the use of coding techniques such as using several interfering carriers, once thought impractical.
Another object of the present invention is to provide a sophisticated coding technique of giving the appearance of several interfering carriers by alternately shifting the frequency of a single interfering carrier.
Yet another object of the present invention is to provide a sophisticated coding technique of modulating an interfering carrier with one or more modulating frequency signals.
Another object of the present invention is to produce a system having a high degree of security in which picture degradation, when removing an interfering carrier or carriers, is minimized. High efficiency and narrow band sophisticated decoders reduce the loss of video information to an insignificant amount.
Still another object of the present invention is to minimize temperature and aging effects by using stable, solid state technology. The critical elements of the decoder are constructed of Quartz or possibly other very stable materials such that no significant drift due to temperature and aging is anticipated.
Still another object of the invention is to provide a television signal transmission system having a high degree of security using high-technology, solid state manufacturing technology which allows batch processing at low cost but provides high security due to the lack of access to this technology by the typical pirate.
Still another object of the present invention is to provide a highly secure television signal transmission system using Solid State Quartz technology to produce, at low cost, highly selective decoders which cannot be reproduced by using standard, discrete circuit components.
Still another object of the invention is to provide means to compensate for the effect of the filters on the video quality.